The present disclosure generally relates to geothermal wells, and, more specifically, to methods for removing geothermal scale formed from a source of geothermal fluid.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. More specific examples of illustrative treatment operations can include drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Scale deposits or “scaling” can represent a particular issue during various subterranean operations. In production wells, such as those producing a hydrocarbon resource, scale deposits can decrease a subterranean formation's permeability and lessen its production capacity and/or rate. Silica scales can be particularly problematic in this regard due to the extreme insolubility of silica and certain silicate species. Hydrofluoric acid or a hydrofluoric acid-generating compound are generally needed to remove silica scale. Various silica scale control additives are also available to limit the initial deposition of silica scale.
Scaling can be an especially problematic issue in geothermal wells and their associated equipment. As used herein, the term “geothermal well” refers to a well structure that establishes a fluid connection between a geothermal fluid and the earth's surface. As used herein, the term “geothermal fluid” refers to a formation fluid that is heated within a subterranean formation by a geothermal heat source. Geothermal fluids can be liquids or gases, such as geothermal brines or geothermal steam. Although geothermal fluids can represent a source of clean energy once they are brought to the earth's surface and transformed into electrical power, they can dissolve high concentrations of a wide range of chemical components, particularly metal compounds, at the fluids' high initial downhole temperatures. The dissolved components can present a number of difficulties, as discussed hereinafter.
As geothermal fluids exit the geothermally heated portion of the subterranean formation and cooling occurs, the solubility limit of the dissolved components can be exceeded and geothermal scale can form. If deposits of geothermal scale are not removed or prevented from forming, a number of deleterious consequences may result, including plugging of the well annulus, pipes, or the formation porosity. Scale-induced damage to downhole tools and surface equipment may also render the tools and equipment inoperative. Corrosion of metal goods in contact with a geothermal fluid can also present an additional difficulty. Furthermore, geothermal scale can impact the efficiency of heat exchangers used to withdraw thermal energy from the geothermal fluid, thereby decreasing the fluid's capacity for energy production.
Geothermal scales can have an exceedingly complex and variable chemical makeup. Even slight temperature differences or chemical content variability within a geothermal fluid can produce geothermal scale deposits having vastly different characteristics and compositions. As a result of this complexity, it is often not easy to predict the outcome of a geothermal scaling process, other than knowing that geothermal scaling is likely to occur. Moreover, geothermal scales can be very dense and non-porous because of their high temperature deposition conditions, often forming a crust-like deposit with a low surface area. These factors in combination with one another can make geothermal scales very difficult to remove.
A number of geothermal scales can contain a siliceous material, related to those found in silica scale. Geothermal scale deposits differ significantly from typical silica scale, however, due to the morphological properties of geothermal scale resulting from its extreme deposition temperatures and co-present metal-derived scale components. For example, the extremely dense and crust-like nature of geothermal scale can differ considerably from the amorphous silica or silicate deposits produced when acidizing a siliceous formation.
The metal-derived components of geothermal scale may be present alone or in combination with a siliceous material. In either case, the metal-derived components of geothermal scale can be problematic for the reasons noted above. Many of the metals present in geothermal scale are not commonly encountered in other scale types. Metals are commonly present in geothermal scale in the form of metal carbonates or metal sulfides. Metal sulfides can be particularly problematic due to their extreme insolubility.
As indicated above, the removal of geothermal scale can be very problematic. The density and low surface area of geothermal scale can often make it difficult to achieve sufficient chemical interaction with a treatment fluid in order to promote scale dissolution. In addition, the chemical complexity and variability of geothermal scale can make it difficult to develop a suitable descaling treatment protocol. One example of a descaling fluid presently in use for removal of geothermal scale is a 4:1 mixture of hydrochloric acid and hydrofluoric acid. However, this descaling fluid presents significant corrosion issues itself and can be costly to dispose of once spent. In addition, in order to support its use, significant cooling of the geothermal well is often required, again adding to treatment time and costs. As an alternative to chemical methods, physical removal of geothermal scale can also be conducted (e.g., by techniques such as scraping, scratching, reaming, hydrojetting, pulverizing or the like), but these techniques can be problematic to implement downhole and may mechanically damage downhole components if not performed carefully.